A submarine is a ship that can operate both on the surface of the water and completely submerged. In order to avoid detection by radar, surface ships and air patrols, a submarine is usually submerged. Modern submarines have the capability of remaining submerged for long periods of time. In fact, a modern submarine can circumnavigate the earth while running submerged. Thus, modern submarines may complete large portions of their missions while being submerged. In order to communicate or check the course of their own submarines, Navies want to determine the current location of their own submarines, without revealing the submarine's present location. Navies also want to determine the location of foreign submarines without revealing the location of the Navy's fleet.
One method utilized by the prior art for achieving the foregoing involved the placing of active and passive buoys in the water. The active buoys projected pulses sound into the water. These sound pulses would travel through the water until they hit an object, at which time the sound pulses would be reflected by the object and possibly detected by the passive buoys as an echo. The speed of sound in water is known. Thus, the range and bearing to the unknown object may be determined by triangulating the echoes received by three passive buoys.
For an echo produced by a submarine to be detected by passive buoys, the echo must be of greater strength than the other interfering echoes. The interfering echoes are caused by the noise produced by the passive buoys or by the reverberations produced by the sound generated by the active buoys. Reverberations are all the echoes returned to an active sonar system from the ocean itself. This includes the suspended marine organisms in the ocean as well as the irregularities of the ocean's bottom. A sonar operator will usually hear reverberations as quavering rings and echoes which directly bounce off submarines as pings. Hence, the sonar operator usually was able to distinguish between reverberations and echoes that were directly reflected by submarines.
Unfortunately, echoes do not travel in straight paths in the upper ocean where the active buoys are located. Echoes travel in the upper ocean in curved paths. Thus, the echoes produced by the submarine were usually quite weak (poor target illumination) and it was difficult to locate the submarine. Even though echoes travel in curved paths, it was possible to locate the submarine by triangulation if conditions were just right, i.e. the buoys and submarine must be in the right location for the sound which was produced by the active buoys to be reflected by a submarine and detected by a passive buoy. If no echoes are reflected by a submarine and then detected by the passive buoys, the submarine is not illuminated and its location is unknown. Sometimes, due to the location of the buoys and the submarine, there was an ambiguity in the location of the submarine. According to the detection buoys, the submarine was in two places at the time (physical impossibility). Since the sonar usually did not know the present location of the deployed buoys and he was unable to redirect the sound generated by the active buoys, it was very difficult to resolve the above ambiguity. Other disadvantages of the foregoing systems were that: buoys had a range of approximately one mile and as the submarine traveled the submarine might travel away from the buoy necessitating the dropping of another buoy so that the transmission between the aircraft or surface ship and the buoy might be continued; the active buoys were heavy; if the buoys were carried by an aircraft, the buoys would require a large amount of space and add to the weight of the aircraft which would reduce the amount of other equipment the aircraft would carry and/or reduce the aircraft's range; and the buoys were expendable which meant that the location of a submarine was relatively expensive.